It can be a one time thing that just vanishes, or it can be the indicator of some bigger problem. I would, first of all, make sure that googlebot has been succesful at retrieving urls in recent weeks from your server. The standard resaon for ur-only listings is that the url could not be successfully spidered, usualy over several attempts.
Here's a thread from earlier in the year. Things may have shifted a bit in some of the areas discussed (this was at the very beginning of the Big Daddy rollout) but still you may find some food for thought.